As can be imagined scanning is an important aspect of space science. In addition to their use in scanning the earth and other planets, x-ray, gamma-ray, and similar scanning instruments perform important functions in space exploration, Examples are sensors, telescopes and electronic devices on platforms such as a space shuttle, a space station, on experimental balloons, and on free-flying spacecraft.
Three general types of scan patterns are well known. The first is a circular scan, in which the line of sight is repeatedly traced in a circle. The second type of scan pattern is a line scan. Here the line of sight is linear, with the scanning device moving back and forth in a line. The third form of scan pattern is a raster scan, which is based on a line scan further complemented with some relatively slower motion, usually perpendicular to the original line scan.
To achieve the scan pattern some means must be provided which impart to the payload an oscillatory motion. Such drive means are generally known, particularly in space exploration. Various forms of machines or apparatus have been employed for controllably conferring on scanning instruments predetermined scan patterns. They include control moment gyroscopes, reaction wheels, torque motors, reaction control systems, and various combinations of such apparatus. However, one disadvantage of utilizing such scan generating devices is power consumption. For this reason, whether scanning a ground based, space based, or balloon borne gimballed payload a preferred drive means which is particularly effective is a rotating unbalanced mass or RUM device. This device is the subject of my U.S. Pat. No. 5,129,600. RUM devices are a new and efficient way to generate scans in gimballed payloads such as x-ray telescopes or other scientific instruments.
A RUM device consists of a mass, m, on a lever arm r, located at a distance, d, from the center-of-mass of the gimballed payload on which it is mounted. The mass is driven at a constant angular velocity .omega. which produces a cyclical centrifugal force m.omega..sup.2 r on the payload. This force, in turn, produces a cyclic torque, about the payload center-of-mass, with an amplitude of m.omega..sup.2 rd. Two RUM devices are required to scan gimballed payloads. They are mounted on each end of the payload and they rotate 180.degree. out-of-phase producing a cyclic torque couple with an amplitude of 2 m.omega..sup.2 rd.
RUM devices are superior to previous scanning apparatus in terms of power, weight, cost, and accuracy, but there are still certain disadvantages accompanying their use. Even though power requirements are less than in other scan generating devices there is still room for improvement. RUM devices require an auxiliary control system to position and reposition the scan pattern relative to a target or a number of targets. The auxiliary control system is also required if a raster scan pattern is to be generated by the RUM device. It confers on the payload a slow complementary motion perpendicular to the line scan to form the raster scan.
All auxiliary control systems known to be suitable for use with RUM devices require the use of some type of feedback control. Generally rate and position sensors are mounted on the spacecraft, the payload, or the payload gimbals. Spacecraft or payload mounted sensors include sun sensors, star trackers, and rate gyros. Gimbal mounted sensors include encoders, resolvers, and tachometers. The outputs of these units are sent to a control computer where they are compared with calculated rate and position commands that correspond to the desired scan pattern. Any differences between the actual and the commanded rate or the current and desired position result in torque commands to the auxiliary control actuators. The spacecraft or payload rate and position commands in the control computer require synchronization with the position and rate commands of the RUM devices in order for the RUMs and the auxiliary control system to work together synergistically. It will be appreciated that this is not a simple operation. To accomplish it the auxiliary control system usually includes torque motors, tachometers and resolvers on the gimbals, as well as a two axis sun sensor and rate gyro on the scanning device. These systems are obviously complex, and consume power saved by the RUM devices. It can be seen then that there is room for improvement even in RUM actuated scanning devices. Such improvements are provided herein.